1. Technical Field
The present invention relates generally to the field of electrical test equipment and more particularly to the field of operability testing for voltmeters and/or similar devices at their point of use.
2. Background Art
Voltage is a term used to describe the amount of energy or electromotive force available to move a certain amount of electrons from one point to another in an electrical circuit, which results in a difference in electrical potential between two points. For most purposes, voltage can be characterized as one of two types, alternating current (AC) or direct current (DC) voltage. Both of these voltage types are commonly used to supply power to various electrical circuits and electrical equipment. A conductor is a material or object that offers a pathway for electrons or current to flow from one point to another in the presence of an applied voltage.
When voltage or electrical pressure is exerted upon a conductive closed loop or circuit, current will flow within the circuit. However, if voltage is applied to an open circuit no current will flow. Regardless of whether or not current flows in an open or closed circuit, both circuits are “energized” by the application of voltage and the voltage differential between two points in the circuit. Similarly, two or more conductors, with an earthen or equipment ground (i.e., reference ground) considered to be one of the conductors and having a voltage differential between the conductors, are considered energized. Resistance is a closely related concept that describes a material or object with physical properties that reduce or restrict the flow of electrons or electrical current in a conductor. Current and resistance are generally inversely proportional in that the higher the level or measurement of resistance in a given circuit, the lower the corresponding current flow in that circuit will be. For purposes of this discussion, the terms “electrical circuit” or “electrical equipment” may be broadly interpreted to mean any type of electrical device or mechanism which is capable of conducting current when a voltage is applied to the circuit.
An appropriately configured electrical instrument can be used to determine whether or not there is a difference in electrical potential between two given points in a circuit. The electrical instrument used will generally indicate a difference in potential by providing some type of feedback to the user of the electrical instrument. For example, a visual indication such as an illuminated light bulb, an audible sound, or a combination of the two may be employed. Several electrical instruments are currently available such as audio emitting instruments containing piezo electric sound or tone generators or light emitting instruments such as neon test probes that indicate voltage. Additionally, voltage measuring meters such as the “Simpson” meter, solenoid driven voltage meters such as the “Wiggins” meter, and various digital voltmeters are also available for this use.
A voltage measuring meter or voltmeter can be used to perform at least two functions when there is a difference in electrical potential in a given circuit. First, the voltmeter can be used to indicate the presence of voltage and second, to determine the magnitude or amount of the difference in voltage potential. For purposes of this discussion, the terms “electrical testing apparatus” may be broadly interpreted to mean any type of electrical instrument which is commonly used to determine a difference in electrical potential.
Although the human body varies from person to person, it is generally accepted throughout the electrical industry that a level of around 500ma of current passed through the heart can stop the heart and cause severe injury or, in some cases, death. A certain level of voltage will be required to push the current through the resistance of the human body as described by Ohm's law where voltage/resistance=current. When a technician uses an electrical testing apparatus against a known voltage source, the technician maybe taking a risk that is more significant than the risk of working on a circuit without testing the circuit to determine if it is energized or not. For example, in attempting to verify the operability of his equipment, the technician might use a known 480V 100 A source when the actual circuit to be worked on is a 50V 5 A circuit. In this situation, the technician is in greater danger from the current associated with the 480V source than the 50V circuit to be worked on.
An electrical testing apparatus, such as a voltmeter, is generally considered to be a very high resistance device so that very little current flows through the electrical testing apparatus when determining if a difference in potential exists. Therefore, a standard electrical testing apparatus consumes very little power during use. To use the electrical testing apparatus, a first electrical sensing test probe or lead will be applied to one point in the circuit and a second electrical sensing test probe or lead will be applied to a second point in the circuit. With the electrical testing apparatus activated, an illuminated light or audible sound or voltage indication seen on a voltage meter will generally signify that there is a measurable difference in potential between the two points being tested. In this fashion an electrical testing apparatus can be utilized to determine whether or not a circuit is energized or de-energized and an appropriately configured voltage measuring meter can determine the magnitude of the voltage present.
An electrical testing apparatus, such as a voltmeter, may be routinely used in the electrical and power generation industries to facilitate equipment diagnostics and repair. In these applications, a worker will utilize a voltmeter to ensure a piece of equipment or a circuit is de-energized prior to disassembling or working on the equipment since working in an energized environment can lead to undesirable consequences. An electrical testing apparatus that does not provide an accurate indication or reading of the electrical potential in an electrical circuit can inadvertently lead a user to believe that a circuit is de-energized when, in fact, the circuit is energized. This situation is quite dangerous and can lead to many hazards with varying consequences, ranging from the very minor to the very serious, including damage to electrical equipment, shocks and bums or even death for the user of the faulty electrical testing apparatus as well as his/her co-workers.
In an attempt to avoid the dangers described above, the Occupational Safety & Health Administration (OSHA) has adopted certain regulations including Standards 29 CFR under which “Selection And Use Of Work Practices 1910.333” has been incorporated. Paragraph 1910.333(b) (2) (iv) (B) states: “A qualified person shall use test equipment to test the circuit elements and electrical parts of equipment to which employees will be exposed and shall verify that the circuit elements and equipment parts are de-energized. The test shall also determine if any energized condition exists as a result of inadvertently induced voltage or unrelated voltage backed even though specific parts of the circuit have been de-energized and presumed to be safe. If the circuit to be tested is over 600 volts, nominal, the test equipment shall be checked for proper operation immediately after this test.”
For compliance with this directive, an electrical testing apparatus such as a voltmeter is commonly used to test the circuit elements and electrical parts of the circuit or electrical equipment to verify that they are de-energized prior to any activity with the circuit or equipment that might expose a worker to potentially dangerous level of voltage. This is known throughout the industry as a “test before touch” philosophy. Even though the OSHA guidelines specifically point out that the test equipment used to verify voltages above 600 volts need only be checked for proper operation immediately after testing for energized circuits has occurred, many companies have adopted this safety practice for all voltages in excess of 50V.
Before working on a circuit, a common “safe working practice” is to check the functionality of the electrical testing apparatus on a known energized voltage source to verify that the electrical testing apparatus is in fact operational. These checks also verify that the electrical testing apparatus's test probes and test leads are functional. The electrical testing apparatus is then used to check and verify whether or not the circuit and/or equipment to be worked on is de-energized. As a last check to verify it did not fail during the circuit testing, the electrical testing apparatus is again checked against the known energized voltage source. This testing procedure is commonly referred to as a live-dead-live (LDL) check.
Currently known techniques for performing the LDL check of an electrical testing apparatus generally require that the user locate a known and quantified energized source and verify that the electrical testing apparatus is operating properly by testing it on the known voltage source. The user then uses the electrical testing apparatus to check the circuit or equipment to be worked on to verify it is de-energized, as evidenced by the visual or audible indication provided by the electrical testing apparatus. Finally, before starting to work on the circuit/equipment, the user re-verifies that the electrical testing apparatus is operating properly by re-checking it against the known voltage source. If the electrical testing apparatus operates properly during the final check against the known voltage source, then the user would know that the electrical testing apparatus had not suffered a failure during the testing process. If the LDL check is performed correctly, with properly functioning equipment, the possibility of accidental exposure to electrical energy can be minimized or eliminated.
The known techniques and methods of performing a LDL check of an electrical testing apparatus set forth herein can be very time consuming and, in certain circumstances, may provide a false sense of security to the user. For example, if there is an intermittent problem with the electrical testing apparatus or its test leads and no voltage is indicated during the LDL check, the user may not know if the circuit is de-energized or if the electrical testing apparatus is simply malfunctioning. Additionally, if the circuit or equipment to be worked on is located in an area where a known voltage source is not present or readily available, the user must leave the area to verify the electrical testing apparatus is operating properly. Accordingly, in cases where both AC and DC voltage may need to be verified, the LDL process may have to be repeated twice, so that the user can verify that the electrical testing apparatus is operating properly for both types of voltage. This technique/methodology is obviously inconvenient and time consuming and, in some cases, may tempt the user to skip the LDL checks entirely. This decision could lead to undesirable consequences, as the unverified electrical testing apparatus then becomes the sole barrier in preventing personnel injury and/or equipment damage.
In addition to the obvious inconvenience, the electrical testing apparatus could theoretically pass an LDL check but still be inoperative at the time of testing the circuit or equipment. For example, due to an intermittent problem that presents itself only while moving from the location where the initial electrical testing apparatus verification was performed to the work location, a faulty electrical testing apparatus could still indicate the circuit/equipment is de-energized when it is in fact energized. The user may attempt to minimize this possibility by carrying a portable voltage source to the work area to perform the LDL checks. This procedure requires that the user carry a portable AC and/or DC source, which is often large and heavy, and possibly an extension cord to the work area. He or she may also need to locate a reliable power source to feed the portable power unit to avoid inaccurate results while performing the LDL checks. In addition, many industrial and commercial facilities utilize circuits and/or equipment that are located in somewhat hazardous environments. Normally, for safety and environmental reasons, every effort is made to minimize both the amount of time spent in the hazardous work area as well as the amount of test equipment taken into the work area. Once again, this technique/methodology is obviously inconvenient and time consuming.
As shown by this discussion, while certain techniques currently exist to perform operability and LDL tests using electrical testing apparatuses, challenges still exist and certain limitations remain. Accordingly, without improved apparatus and methods for performing operability and LDL checks, inadvertent exposure to live electrical current will remain a very real possibility.